Our work on the mechanism of mineral absorption has culminated in the development of a combined model for Ca and Pi absorption in the gastrointestinal tract. Tracer methodology is used to analyze segmental absorption of Ca and Pi from a spontaneously propelled meal, along the gastrointestinal tract. The data were analyzed using a compartment model to allow estimation of transit and absorption rate constants, segmental mean residence times maximal absorption rates and the % of total absorption for Ca and Pi. The data were analyzed using a compartment model to allow estimation of transit and absorption rate constants, segmental mean residence times maximal absorption rates and the %of total absorption for Ca and Pi. The simultaneous model for Ca and Pi absorption that we have developed as part of this collaborative project represents important tools for our use in understanding mineral absorption in man.